In recent years, realistic expression of materials has become an important factor in movies, games, and animation, and various techniques for realistic expression have been developed. However, it takes a lot of time to utilize various techniques, and the cost of utilizing various techniques also increases.
Especially, in the case of a material in which light is reflected and diffused, such as the human skin, the realistic expression is not achieved. In other words, it is very difficult to reproduce human faces more realistically in computer graphics because the multi-layered skin absorbs and scatters light differently at each layer.
Currently, with the development of a 3D scanning technology, it is possible to model the 3D character at a level similar to the real face. In addition, for more realistic expression of the real face according to the development of the infrastructure, a method of applying the physical scattering effect of light to a real human face has been developed. The method uses a method of dividing the skin into several layers and applying reflection and diffusion of light according to the divided layers.
For example, a bidirectional reflectance distribution function (BRDF) method is used to express light dispersion according to a human skin layer using a three-dimensional engine of computer graphics. The BRDF method is mainly applied to the expression of specular, and then the diffuse effect through a diffusion profile is calculated in the lower skin layer.
Referring to FIG. 1, layers of the real human skin 10 is divided into an oil layer 12, an epidermis 14, and a dermis 16. Incident light 20 incident to the skin 10 is reflected at the oil layer 12, that is, the surface of the skin layer 10 from the light source (not illustrated) and generate reflected light 22 or scattered by the inside of the skin 10, that is, the epidermis 14 and the dermis 16.
Rendering for realistic skin expression means a technique for expressing the physical diffusion and dispersion effects of light sources according to each layer of the skin on a three-dimensional screen and expressing a real skin to which a physical concept is applied in real time. The rendering means a process of creating a three-dimensional image by injecting a sense of reality into a two-dimensional image by considering external information such as a light source, a position, and a color.
Since the oil layer 12 is finely rough on the surface of the skin, the reflection by the oil layer of the skin is not a reflection of a mirror. The oil layer 12 has an incident angle and an angle of reflection, and uses a bi-directional reflection distribution function (BRDF) scheme for expressing the oil layer. This is applied in a shader function in a specular form from the last render pass of the rendering.
In order to process a light scattering effect in the epidermis 14 and the dermis 16, a technique for expressing a physical scattering called a diffusion profile is applied by creating a plurality of irradiance textures. The diffusion profile is a method of estimating scattering of the light when the light is diffused below the semi-transparent surface having a high scattering degree. For example, the diffusion profile is a function of a distance and an angle from the center of the light and notifies how much light is emitted. This diffusion profile is very dependent on the color. For example, red is scattered farther than green and blue. Scholars have implemented a spectral model that theoretically simulates 150 color areas on the skin. However, this is not realistic because of being subjected to many render passes in reality, and generally, rendering is performed through RGB light diffusion.
This diffusion profile needs to create irradiance textures sequentially in accordance with the characteristics of an algorithm, and in the final rendering step, the skin is realistically expressed by a method of synthesizing the irradiance textures by applying a gaussian filter. That is, in the method of dividing the skin layer and applying each physical diffusion profile, a plurality of irradiance textures to be expressed are created, and the gaussian filter is applied and a linear combination method is used. In this process, the irradiance-related textures need to be created sequentially and linearly. For this reason, a lot of time is invested in the creation and change of the irradiance textures, and there is a limit to the fast rendering, and thus, there is a limit to the creation by a parallel process. Further, during this complex processing, especially in the process of creating the plurality of irradiance textures and filling information, a load is generated.
Therefore, it is almost impossible to express the realistic skin for the three dimensional characters in real time using the BRDF method.
Further, in order to reduce the load caused by the realistic skin expression of the three dimensional characters, a level of detail (LOD) technique is used or a method of reducing the irradiance processing step is used. However, in such a method, the expressed three dimensional characters are also expressed by a similar skin texture rather than the realistic skin. In addition, when a plurality of face characters is realistically expressed in one content, there is a problem in the processing speed, and thus, the load problem becomes more remarkable.